Diffusion and chemical reaction as limiting factors in CO2 equilibration in lungs.

Blood CO2 exchange involves at least five separate diffusion and/or chemical reaction processes occurring simultaneously, the rates of several of which have been measured in vitro. Estimation of the influence of the velocity of a single process on the overall rate of CO2 exchange requires calculations using a mathematical model of the system. Computation shows that inasmuch as there is no carbonic anhydrase in plasma, there should be a slow readjustment of plasma pH after blood exchanges CO2 in capillaries. However, there appears to be a carbonic anhydrase in addition to the one in red blood cells that is available to intracapillary fluid in the lung and that accelerates equilibration of the plasma bicarbonate buffer system. This carbonic anhydrase may be in the capillary endothelial cells.